President Obama’s warning to Syria that the use of chemical weapons would cross a “red line” for the administration drew criticism Tuesday, with experts saying that the remark could provide President Bashar al-Assad cover to continue battling the opposition with tanks, warplanes and other conventional weaponry.

Obama on Monday used some of his most specific language yet to warn Assad not to use or move Syria’s stockpile of chemical weapons, saying that such an action would change his “calculus” about the possibility of a U.S. intervention.

“We have been very clear to the Assad regime, but also to other players on the ground, that a red line for us is [if] we start seeing a whole bunch of chemical weapons moving around or being utilized,” Obama told reporters. “That would change my calculus.”

Officials said the warning was intended as a reminder to Syria of the limits of American patience. But experts said they feared that the president’s remark would be interpreted in Damascus as a signal that the United States would not take stronger action even in the face of the Syrian military’s use of heavy weaponry in a conflict that is in its 18th month.

“I don’t like his formulation at all. It inadvertently tells the Syrians they can get away with anything but chemical weapons,” said Robert Satloff, executive director of the nonpartisan Washington Institute for Near East Policy.

The unusually direct U.S. warning on Syria’s weapons of mass destruction echoes discussion of the “red lines” that Iran must not cross in developing its disputed nuclear program.

Iran has carefully assembled nearly all the resources it needs to build a bomb without crossing the boundaries that the United States and others have drawn — such as the expulsion of international nuclear inspectors or the enrichment of uranium to levels that would fuel a nuclear bomb.

Christopher Chivvis, a specialist on international interventions at the Rand Corp., agreed that Assad may hear only what he wants to hear.

“It could be read the opposite way,” Chivvis said. He added, however, that Obama might be laying a marker for possible action if U.S. intelligence indicates that Assad does intend to use chemical weapons.

The United States is monitoring at least four known chemical weapons sites in Syria. In addition to concerns about the Assad government’s use of the stockpiles, U.S. officials have expressed fears that chemical weapons could fall into the hands of the Lebanon-based Hezbollah militia or affiliates of al-Qaeda.

A senior Western official said that “no change” has been detected in the location or protection of the Syrian stockpiles and that there is no indication that the Syrians are readying the weapons for use. The official spoke on the condition of anonymity to discuss intelligence matters.

A year after declaring that Assad must leave power, the Obama administration has continued to resist any direct military involvement in Syria’s escalating civil war, saying that even the limited provision of arms could inadvertently make the situation worse.

The administration has provided some help to Syrian rebels but has not endorsed any military intervention inside Syria or a protective “no-fly” zone that would involve U.S. resources.

Responding to Obama on Tuesday, Syrian Deputy Prime Minister Qadri Jamil said the West is looking for an excuse to attack.

“If this excuse does not work, it will look for another excuse,” he said in Moscow, where the Russian government was hosting a delegation of Syrian officials, including the head of the Assad government’s newly established Ministry for National Reconciliation.

Jamil added: “Regarding Obama’s threats, they are media threats to be used in the media campaign in readiness for the coming elections.”

In Syria, Assad’s forces evacuated two security installations along the Iraqi border Tuesday as rebels made gains in the strategically important area after a week of heavy fighting, opposition sources told the Reuters news agency. Most of the day’s fighting appeared centered in the suburbs of Damascus, which have witnessed a dramatic uptick in violence over the past month.

A Turkish official said that about 2,500 people fleeing the bloodshed in Syria have entered Turkey over the past 24 hours, one of the highest daily refugee flows in recent weeks.

Rebels have seized a large stretch of territory in northern Syria near Turkey, forcing a new international debate about the value of intervention in the conflict. U.S. officials have discussed a number of possible options, including a safe zone for refugees and a protected corridor to supply the rebels inside Syria.

Beyond any foreign policy calculations, the Obama administration thinks there is little to be gained in an intervention whose length and outcome would be impossible to predict, and it sees no public appetite for U.S. involvement in yet another foreign war.

The chemical weapons warning was clearly intended as a “deterrent, more than any signal that the United States is going to interfere militarily at this stage,” said Marwan Muasher of the Carnegie Endowment for International Peace.

In the Middle East itself, suspicion of U.S. motives prevails. Obama’s popularity is at a low ebb despite his efforts at outreach. Negative memories of the invasion and occupation of Iraq far outweigh the positive results of the more recent intervention in Libya. Other than the Persian Gulf states, said Muasher, a former foreign minister of Jordan, “the images of Iraq are stronger” than concern about the brutal crackdown in Syria and possible spillover into neighboring states.

Although Obama’s comments provided new clarity about the administration’s calculations on Syria, its public pronouncements on Iran have been more vague.

The president has said that “all options” are on the table, but he has never set an explicit trigger for military action against Iran.

Critics have accused Obama of miscalculating by not explicitly drawing a red line beyond which Iran would guarantee a strike on its facilities. Obama’s defenders say those red lines are already well understood by Iran and would only limit his bargaining power in the talks, now stalled, to get the Islamic republic to back off the nuclear brink.

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